Recently, the scaling-down of a semiconductor device has been in progress. To that end, it is needed to form a finer circuit pattern on a surface of a substrate for a semiconductor device. As the circuit pattern is formed more finely on the surface of a processing target substrate, it becomes necessary to manage and control as foreign substances particles having small diameters of several tens of nanometers (e.g., 30 nm to 100 nm), which have conventionally been treated as being nonproblematic, because such small-sized particles tend to cause an open circuit or damage on device characteristics in a semiconductor device, for example.
As the diameters of the particles as foreign substances get smaller, an adsorption strength by which such small-sized particles are adsorbed onto a member, e.g., a processing target substrate, may be dominantly affected by an electrostatic force. Accordingly, it becomes important to prevent the charged particles from being attached on a member, e.g., a processing target substrate, in a substrate processing apparatus employing an electrostatic chuck which attracts and holds the processing target substrate to perform the plasma-etching on the processing target substrate.
FIG. 6 is a graph showing relationships between a size (diameter) of a particle and an adsorption strength thereof. In FIG. 6, the vertical axis indicates an adsorption strength (deposition velocity) (cm/s), and the horizontal axis indicates the size of the particle. It can be seen that, as the diameter of the particle gets smaller, especially, equal to or smaller than about 100 nm, the adsorption strength is dominantly affected by the electrostatic force as described above. Accordingly, to prevent the particles from being attached on the processing target substrate or the like, it is essential to maintain the particles not to be charged or such static electricity not to be accumulated in the substrate processing apparatus.
As described above, the static electricity may cause the particles to be adsorbed onto the processing target substrate or the like and, furthermore, may cause electronic components to be damaged. For example, a semiconductor device may be damaged by the static electricity of about 1000 V. Moreover, when charges in a charged processing target substrate are released to other parts, in a substrate processing apparatus employing an electrostatic chuck for attracting the processing target substrate thereon, the processing target substrate may be damaged or a discharge trace may remain in the processing target substrate. As a result, the production yield may be lowered.
FIG. 7 shows how a substrate processing apparatus is affected by static electricity. As shown in FIG. 7, the substrate processing apparatus includes a process module 71 for performing a predetermined process on a processing target substrate (hereinafter, simply referred to as “wafer”); a loader module 72 serving as a transfer chamber that transfers the wafer; a front-opening unified pod (FOUP) mounted to the loader module 72 and accommodating the wafers; and a load-lock module 74 connecting the loader module 72 to the process module 71. A processing vessel (hereinafter, referred to as “chamber”) of the process module 71 is provided with a gas exhaust path 75 and a dry pump 76 which serves to exhaust a gas from the vessel.
In such a substrate processing apparatus, the following problems may be caused by static electricity accumulated therein. That is, by the static electricity accumulated in the FOUP 73, floating particles therein are charged, thereby being adsorbed onto a wafer W. Furthermore, by the static electricity, a discharge may be generated between the FOUP 73 and a wafer accommodated in the FOUP 83, thereby causing a discharge trace to remain on the wafer W. The inside of the load-lock module 74 is repeatedly pressurized to an atmospheric pressure state and depressurized to a vacuum state. For that reason, static electricity may be easily accumulated in the load-lock module 74. The accumulated static electricity may cause the particles to be adsorbed onto the wafer or the like.
Moreover, when a wafer W is exposed to a plasma or the wafer W is attached on or detached from the electrostatic chuck in the process module 71, the wafer W may be charged, thereby causing particles to be adsorbed thereonto. Further, while the gas inside the chamber of the process module 71 is exhausted through the gas exhaust path 75, reaction products produced by a chemical etching reaction or the like of a processing gas and unreacted residual substances may be charged by static electricity generated by friction with an inner wall surface of the gas exhaust path 75. As a result, the reaction products and the unreacted residual substances may be attached and deposited on the inner wall surface of the gas exhaust path 75. As such deposits are gradually increased, the gas exhaust path 75 may be eventually blocked.
In addition, due to static electricity accumulated in the loader module 72 serving as a substrate transfer device, the particles therein are charged, thereby causing a particle contamination problem that the charged particles are attached onto the wafer W.
For example, Japanese Patent Application Publication No. 2002-353086 (JP 2002-353086 A) discloses a technique to prevent in advance the occurrence of the problems caused by static electricity generated in various components of such a substrate processing apparatus.
In JP 2002-353086 A, a charge-neutralizing device is provided in an air lock chamber serving as a passage through which a processing target substrate, e.g., a wafer, is transferred between a processing chamber and the outside. The charge-neutralizing device generates an ion flow inside the air lock chamber to charge-neutralize ionic particles (remove static electricity). Then, a gas inside the air lock chamber is vacuum-evacuated, thereby exhausting and removing the particles from the air lock chamber.
Moreover, JP 2002-353086 A also discloses that, after the particles attached onto the inner wall of the air lock chamber are exhausted and removed from the air lock chamber, the wafer is loaded into the air lock chamber. Then, the charged particles attached onto the wafer can be electrostatically adsorbed onto an electrode provided above the wafer by applying to the electrode a voltage, which is determined by considering the charged state of the wafer.
However, in the above-described conventional technique, there is no mention of preventing the particle contamination of a wafer caused by the charged particles in a substrate transfer device (hereinafter, referred to as “loader module”) which transfers the wafer in the atmosphere.
Fine particles included in the atmosphere that is introduced into a main body of the substrate transfer device (loader module) and passes through a downward flow forming unit, e.g., a fan filter unit (FFU), is dominantly affected by an electrostatic force. If such fine particles are charged, they may be attached onto a wafer or the like, thereby causing the particle contamination. Accordingly, it is needed to find an effective method to prevent the particle contamination caused by such fine particles in the substrate transfer device, especially, that have passed through the FFU).